Next-generation long-haul, fiber-optic communications systems are being designed to operate at 100 gigabits per second over distances of 1,000 kilometers or more. Coherent optical receivers have been proposed as an alternative to conventional direct detection receivers for high-speed, fiber-optic systems because, among other reasons, they recover the phase of optical electric fields. When in-phase (I) and quadrature (Q) components of an optical signal are known, exact equalization of linear channel impairments is possible in principle and the effects of nonlinear impairments may be reduced.
The performance of a high-speed coherent optical receiver is affected by signal levels in the receiver, among many other factors. Poor control of signal levels can lead to unacceptably high bit-error rates.
FIG. 1A is a block diagram of a coherent optical receiver. In FIG. 1A, incoming optical signal 105 and local oscillator (LO) optical signal 110 are inputs to optical hybrid mixer 115. The optical outputs of the hybrid are converted to electrical signals by photodetectors 120 and the electrical signals are amplified by transimpedance amplifiers 125 before being sent to digital data receiver 130 for demodulation and decoding.
In real-world applications the power of input optical signal 105 can vary. The signal power may depend on network conditions, link design and other factors. The power can fluctuate temporarily as optical channels in a link are added or dropped, for example. Both single-ended and balanced photodetection schemes lead to transimpedance amplifier input signals that are proportional to Re{ASIGA*LO} where ASIG is the optical signal amplitude and ALO is the optical local oscillator amplitude; * denotes complex conjugation. Thus, variable input signal power leads to variable signal levels in a coherent optical receiver.
A transimpedance amplifier has a limited, and predefined, range of input signal levels for which acceptable output signal fidelity is assured. When the input signal to a transimpedance amplifier is too low, the output signal-to-noise ratio is degraded. Conversely, when the input signal level is too high, the output suffers from harmonic distortion.
The input signal level and the gain of a transimpedance amplifier determine its output signal level. In the receiver of FIG. 1A, the output of transimpedance amplifier 125 is sent to digital receiver 130 and, as described in further detail below, analog-to-digital converters (ADC) in the digital receiver convert input signals into data for processing.
Too small signal input to an ADC leads to quantization errors while too large signals are distorted by clipping. (Some ADC signal clipping is acceptable to keep most of the signal energy in the ADC's linear conversion range especially when signals have high peak-to-average ratios.) These constraints lead to a predefined, acceptable ADC input signal range.
Overall performance of a high-speed coherent optical receiver thus depends, in part, on keeping signal inputs to transimpedance amplifiers and analog-to-digital converters within predefined, acceptable ranges. What are needed are coherent optical receiver designs that provide immunity from fluctuations in input optical signal level or variations in transimpedance amplifier gain.